Vibrational absorption, vibrational circular dichroism, and theoretical studies of methyl lactate self-aggregation...

The infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of methyl lactate in carbon tetrachloride and methanol have been measured in the 1000-1800 cm(-1) region. Noticeable changes due to the solute self-aggregation and solvent-solute intermolecular hydrogen-bonding interactions have been detected in the reported spectra of the 2M methyl lactate solution in CCl(4) and in methanol, respectively. Molecular dynamics simulations and a series of density functional theory (DFT) B3LYP/6-311++G(**)) and single point MP2/6-311++G(**) energy calculations have been performed to identify and to model the explicit hydrogen-bonding interactions between the methanol solvent and the methyl lactate solute and among the methyl lactate molecules. Geometry search and optimization have been performed for the most stable conformers of the methyl lactate dimer and the methyl lactate-(methanol)(N) clusters, with N=1, 2, and 3. The relative single point MP2 energies among conformers are noticeably different from those obtained with DFT for the larger methyl lactate-methanol complexes. The VA and VCD spectra of these complexes have been simulated and compared to the corresponding experimental spectra. From the combined experimental and theoretical VA and VCD studies, it has been identified that both the methyl lactate monomer and dimer are the main species in the 2M CCl(4) solution with 65% and 35% relative abundances, respectively, while the binary (55%) and quaternary (30%) methyl lactate-methanol clusters dominate in the 2M methanol solution, together with a smaller amount (15%) of the methyl lactate monomer. The effects of solute self-aggregation and solute-solvent interactions have been investigated in detail.